1. Field of the Invention
The present invention relates to an improved method and apparatus for obtaining synchronization in a digital communication system, and more particularly, to the encoding and pre-demodulation recovery of synchronization information which is necessary for decoding a modulated block-coded digital information signal.
2. Description of the Prior Art
The modulation and subsequent demodulation of block-coded digital information signals is widely used in various signal transmission or communication systems. One system of particular interest currently is the transmission or communication of high definition television (HDTV) signals. With respect to television signals, as used herein the words "transmission" or "communication" means any communication of the TV signal, such as by broadcasting, narrowcasting or by recording and subsequent playback from a storage medium, such as in a digital VCR. As well known to those of ordinary skill in the art of HDTV television signal processing, digital video information can be processed to develop block-coded digital signals, in an effort to transmit as much information as possible in the limited bandwidth of the transmission channel, while at the same time obtaining as few errors as possible in the recovered and reconstructed HDTV signal. In block coding, a continuous data stream is parsed into sequential blocks of data of prescribed size, 2.sup.k, which are coded as n-bit codes; n is larger than k, which introduces the redundancy required for error detection and correction. In order to increase the efficiency of data transmission, a symbol coder is customarily used to further encode the n-bit binary codes resulting from block coding, coding them as symbols. A symbol coder, for example, would convert each of successive groups of three binary bits of the block-coded signal into a representative one of eight amplitude levels, each amplitude level uniquely modulating the phase of a carrier signal, commonly referred to as Phase-Shift-Keying (PSK) modulation, which is used for transmitting the coded signal in the transmission channel. Alternatively, groups of four bits can be used to control both phase and amplitude modulation of the carrier, as in a 16 QAM (Quadrature Amplitude Modulation) system, for example.
The format of a block-coded signal descriptive of television images basically comprises a data sub-block having compressed digital information (and also usually associated with a supplemental data sub-block containing an ECC or Error Correcting Code) as well as a sync sub-block having synchronization information encoded therein. The synchronization information encoded in the sync sub-block must be recovered in order properly to reconstruct the time sequence of the original digital information which is in the data sub-blocks. As noted above, in order more efficiently to transmit the digital information, the block-coded signal is used to modulate the phase and possibly the amplitude of a carrier used for transmitting data via a transmission medium (e.g., via a magnetic tape recorded by the recording portion of a VCR). Upon recovery of the transmitted signal (e.g., upon VCR playback), synchronization is necessary for proper operation of the apparatus which demodulates the modulated block-coded signal, as well as for proper operation of the symbol and block decoders.
FIG. 1, illustrates in block diagram form a prior art receiver for a transmission carrier modulated in accordance with symbol-coded response to block-coded signals. An equalizer 2 processes the signal to partially correct for transmission-related amplitude and phase distortions, and after such channel equalization the modulated transmission carrier is applied to a demodulator 4 for detecting the modulation of the transmission carrier. A partial synchronization of the carrier is achieved using a PLL (phase-locked loop) 6, to adjust the frequency of a carrier signal for operating demodulator 4 that is generated by a carrier generator 8. There is also an adjustment of the phase of that carrier signal, but carrier phase lock is usually ambiguous in nature. After demodulation the signal is applied to a complex (Real and Imaginary) baseband equalizer 10, where a phase equalization which is more precise than that which was carried out in equalizer 2 is carried out under the control of a carrier phase detector 12. This phase equalization customarily is done with reference to the content of a portion of the received signal that is known a priori at the receiver, so the phase equalization is non-ambiguous in nature. Next, the block-coded signal is applied to a symbol decoder 14, which converts the symbols back into binary bits, in a manner complementary to the symbol coding performed at the coding section of the system. For proper operation of symbol decoder 14, as well known, synchronizing information, such as the precise phase of the symbols is required, for proper sampling of the epoch of the symbols. The generation of this phase information is illustratively shown as being generated by a symbol phase detector 16. Finally, the block-coded signal recovered by the symbol decoder 14 is applied to a block decoder 18, to be converted back into a continuous stream of time-sequential bits. For proper operation of block decoder 18, block synchronizing information, such as the beginning and end of each block, is needed to identify (and thereafter decode) each block; this synchronizing information is customarily contained in the sync sub-block. This block synchronizing information is provided by a sync detector 19 that responds to certain code groups in the output signal from the symbol decoder 14. After identification and decoding of the sync sub-block information, the remainder of the block-coded signal can be decoded.
Thus, current receivers for modulated block-coded signals require a plurality of frequency and phase detectors, and associated feedback and feedforward loops, for synchronizing the demodulating and decoding processing for proper recovery of block-coded information signals. Additionally, some of the circuitry may require complex signal processing, involving real and imaginary components of signal, adding further to the complexity of the synchronizing system. The present invention is directed to a more straightforward method and apparatus for obtaining the synchronization information necessary for recovery of such signals, wherein the synchronization information can be detected directly from the envelope of the modulated transmission carrier.